1. Field of the Invention
The present invention is related to machinery having a relatively movable shaft, where a dynamic seal establishes sealing contact with the relatively movable shaft to retain a pressurized fluid. Examples of relatively movable shafts include shafts that have relative rotation with respect to the dynamic seal, shafts that have relative reciprocation with respect to the dynamic seal, and shafts that have both rotation and reciprocation relative to the dynamic seal. More particularly, the present invention is related to a backup ring that increases the ability of a dynamic seal to withstand high differential pressure while still allowing freedom of relative shaft movement.
2. Description of the Prior Art
Dynamic seals for high differential pressure service are often mounted in a housing, slide relative to a shaft, and provide a sealing function between the housing and the shaft. A high pressure dynamic seal requires a small shaft-to-housing extrusion gap clearance to avoid extrusion damage, yet the clearance must be large enough to avoid heavily loaded shaft-to-housing contact at the bore of the housing that defines the extrusion gap clearance. If heavily loaded shaft-to-housing contact occurs at the extrusion gap of a dynamic seal during relative rotation, the housing assumes the lateral load intended for the bearings. The resulting friction and heat can destroy the dynamic seal, and may damage the housing and the shaft sealing surface in ways that are highly detrimental to the effective life of the dynamic seal. Heavily loaded shaft-to-housing contact can also occur with reciprocating shafts due to misalignment related binding, and can lead to scoring and galling of the shaft sealing surface.
Some of the optimum conditions for high pressure extrusion resistance are:                Small extrusion gap clearance.        Avoidance of heavily loaded contact between the seal housing and shaft, particularly at the region of intended extrusion gap clearance.        Minimal relative radial motion between the housing and the shaft to minimize dynamic changes to the size of the extrusion gap clearance.        Minimal pressure induced deformation (“pressure breathing”) of the extrusion gap clearance.        An undamaged corner between the seal groove and the housing bore that defines the extrusion gap.        
The implications of excessive shaft-to-housing extrusion gap clearance are:
1.) Pressure causes seal material to protrude into extrusion gap clearance.
2.) Runout and pressure breathing flex the protruding seal material.
3.) Cyclic strain destroys the protruding seal material.
4.) Pressure causes more protrusion.
5.) The damage cycle continues until sealing function is lost.
A number of factors prohibit a small extrusion gap clearance and precise shaft guidance in large machinery. Manufacturing tolerances are large, which directly affects the size of the shaft-to-housing clearance, and also increases bearing mounting and internal clearances, which all permit shaft run-out and misalignment. Large components are often subject to significant elastic deformation when exposed to high pressure and large mechanical loads.
Surface speeds can be significant in large diameter rotary equipment, which adds to seal-generated heat. This softens the seal material, and reduces seal extrusion resistance.
The present invention is directed at applications with relative rotation, relative reciprocation, or both. For the purpose of understanding prior art problems, one can study the annular rotary control devices (also known as rotary blowout preventers) shown by U.S. Pat. Nos. 5,588,491 and 5,662,171. These patents are titled “Rotating Blowout Preventer and Method.” The devices represented by these patents expose a dynamic seal to relative rotation, relative reciprocation, and high differential pressure. Because these units have non-floating seal housings, stack alignment must be essentially perfect. However, there is much potential for misalignment between the lower shaft and the seal housing. The shaft for the lower dynamic seal is an extension of a large reciprocating piston that actuates the packer element. This large piston is necessarily mounted with enough clearance for sliding, which adds to its potential for misalignment. The piston is a two-piece assembly, which also adds to the potential for misalignment of the sealing surface. The piston is hydraulically thrust against the packer element to actuate the packer element. The packer element deforms in uneven fashion against a drillstring. The drillstring is in all likelihood misaligned with the rotating control device, and not running true. This imposes severe cocking loads on the reciprocating piston, causing misalignment and run-out that can cause heavily loaded contact at the extrusion gap that is likely to damage the dynamic seal, the shaft, and the seal housing.
As a general conclusion concerning large equipment such as rotary control devices, if clearance is in a useful range for extrusion resistance, it is virtually impossible to guide the shaft so precisely that it will not rub on the bore of a conventional non-floating seal housing. This rubbing creates seal-damaging heat, and may also damage the seal housing and shaft in ways that significantly accelerate rotary seal extrusion damage.
Internal fluid within a rotating control device is often pressurized for various reasons. One reason for internal fluid pressure is to actuate the packer element. Another reason for fluid pressure is to orient the dynamic seal that partitions the internal fluid from the drilling fluid.
The use of dynamic seals for establishing sealing between relatively movable machine components for fluid retention is known. The differential pressure that such dynamic seals can retain is limited by the extrusion resistance of the material used in the construction of the seal, and is also limited by the size of the extrusion gap clearance that exists between the seal housing and the mating relatively movable surface. In the case of dynamic seals designed to provide hydrodynamic interfacial lubrication in response to relative motion, the size of the extrusion gap clearance also influences interfacial lubrication. The backup rings used with static and reciprocating seals are unsuitable for rotary seals exposed to high differential pressure due to liabilities such as high friction, high wear, material flow, and damaging frictional heat.
U.S. Pat. No. 5,195,754, entitled “Laterally Translating Seal Carrier for a Drilling Mud Motor Sealed Bearing Assembly,” and U.S. Pat. No. 6,227,547, entitled “High Pressure Rotary Shaft Sealing Mechanism” disclose axially force balanced seal carriers that follow the often unavoidable lateral motion of rotary shafts while maintaining a relatively small extrusion gap clearance between the seal carrier and the rotary shafts. The seal carriers of these patents require a step change in shaft diameter, and therefore cannot accommodate the significant shaft reciprocation found in some types of rotary equipment, such as the rotating control devices shown in U.S. Pat. Nos. 5,588,491 and 5,662,171. Another problem with the seal carriers of U.S. Pat. Nos. 5,195,754 and 6,227,547 is that seals and seal carriers of different diameters are typically required for pressure staging.
The seal carriers of U.S. Pat. Nos. 5,195,754 and 6,227,547 are subject to the coning problem that is shown in FIG. 3 of Chapter D16 Rev. 0 (Apr. 23, 2012) of the Kalsi Seals Handbook, Rev. 6. Chapter D16 is titled “Axially force balanced, laterally translating arrangements,” and describes the problem as follows: “The portion of a laterally translating high pressure seal carrier that is located between the Kalsi Seal and the face seal experiences a radial pressure imbalance that causes an inward deflection of the seal carrier. This deflection has to be taken into account when designing the journal bearing fit, the extrusion gap fit, and the axial fit of the seal carrier with the surrounding support structure.” The problem with merely taking the deflection into account when designing the extrusion gap fit is that the extrusion gap closes as differential pressure builds, trapping extruded sealing material.
U.S. Pat. No. 6,007,105, entitled “Swivel Seal Assembly” provides pressure staging across a plurality of rotary seals engaging a relatively rotatable washpipe, and provides for simultaneous articulation of the washpipe and a surrounding seal housing to accommodate shaft run-out and misalignment. This simultaneous articulation is made possible by axial pressure balancing of both the washpipe and the seal housing. Because the seal housing expands due to internal pressure, the extrusion gap clearance changes as the sealed fluid pressure increases. Higher differential pressures could be withstood if the extrusion gap clearance could be held more constant.